Fluorescent lamp



Dec. 27, 1960 O. H. BIGGS FLUORESCENT LAMP Filed June 18, 1958 INVENTOR. ORR/[K H- 5/665 ATTORNEY United States Fatent IQ FLUORESCENT LAMP Orrick H. Biggs, Beverly, Mass., assignor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Filed June 18, 1958, Ser. No. 742,876

4 Claims. (Cl. 313-109) 'This invention relates to electric gaseous discharge 'lamps, especially to fluorescent lamps of that type, and -particularly to those containing a gas at low pressure.

Such lamps generally have an electrode of coiled tungsten wire, the middle portion of the coil carrying a coating 'or filling of electron-emitting materials such as alkaline earth oxide, and the remainder being free from such ma- .terial.

I have discovered that the life of the electrode can be :greatly increased, and the discoloration of the lamp reduced, by coating said remainder of the coil with a firmfly-adherent coating of a refractory insulating material, a such as for example, aluminum oxide.

Such a coating is especially helpful in discharge lamps 'of very high output, but is useful also in lamps of more :moderate output, such as the otherwise standard 40-watt ifiuorescent lamps, which are of 1 /2 inch diameter tubing, about 48 inches long. The latter lamps are generally :filled with, in addition to mercury vapor, argon gas at a gpressure of about 2 mm. of mercury. Such lamps already have a rated life of 7500 hours, and by the use of my in vention that life can either be increased by reducing the :argon pressure, or the life kept at the same value and the light output for rated wattage increased by the same :means. The latter will generally be preferable because .the cost of a lamp is generally much smaller than the (Cost of the energy for operating it during normal life, :and in addition, a brighter lamp allows the use of fewer fixtures for lighting an area to a given brightness.

As pointed out briefly above, I have discovered that the electron-emitting coating should be confined to the middle portion of the electrode structure and that the ends of the electrode, that is the portions extending from the middle of the cathode to the lead-in wire, should be coated with a refractory insulating material, for example, aluminum oxide. The coated end portions should be of as small diameter as possible, to prevent interference with the ion stream to the cathode, and should extend out at least about 2.5 mm. from the end of the coil, and preferably at least 5 mm. if the major component of the gas filling in the lamp is neon.

There should be no gap between the end of the electron-emitting coating and the beginning of the insulating coating, and the electron-emitting coating preferably slightly overlaps the insulating coating.

The electrode structure preferably includes a doubly or triply coiled wire with straight legs, that is a structure in which a coil of small diameter is itself wound into coil of larger diameter at its middle portion, while the ends or legs extending outward from said coil are coiled one less time and are thus of smaller diameter. For example, if the middle portion is triply-coiled, the extending legs would be just doublycoiled.

The middle or large-diameter portion of the coil carries an electron-emitting coating, for example a coating of the usual oxides of barium, strontium and calcium, preferably with a small percentage of zirconium dioxide, and the extending legs are coated with a refractory insulating material such as aluminum oxide, titanium dioxide, or the like.

When such a material is used, it can be applied cataphoretically, if desired, and then sintered to give a firmly-adherent coating. The insulating coating can be applied to the ends of the cathode, and the alkaline earth oxide coating then applied to the middle portion.

The insulating coating confines the active or effective portion of the cathode to the middle of the coil, and allows the lead-in wires to be spaced outside the cathode ionization layer. The aluminum oxide acts as an effective insulating material to prevent ion bombardment of the non-effective portion of the cathode. With long legs extending outwardly from the effective portion of the cathode, the greater distance from the latter to the leadin Wires reduces heat conduction losses therefrom and keeps it a more uniform temperature throughout its length.

Other objects, advantages and features of the invention will be apparent from the following specification, in which:

Fig. 1 is a view partly cutaway, of a lamp according to an embodiment of the invention;

Fig. 2 is a top view of the mount structure of said lamp;

Fig. 3 is an enlarged view of the cathode in said lamp.

For convenience, the invention will be described with respect to a high output fluorescent lamp, although it can be used also with lamps of lower output.

In Figure 1 the lamp envelope 1 is a glass tube having a flared stem 2, of the usual type, sealed to its end to close the same. The lead-in wires 3, 4, are sealed through the press 5 of said stem and extend into communication with the interior of said envelope 1. Just above the press 5, a flat metal disc 6, which can be of aluminum, is welded to the lead-in wire 4 by the tab 7, extending from said disc. The ceramic bushing 8 extends through the disc 5 to insulate lead-in wire 3 therefrom. Bent outwardly above the disc 6, the lead-in wires then extend forwardly into the envelope 1 in a direction parallel to the longitudinal axis of said tube.

The filamentary coil 9 comprises the doubly coiled legs 10, 11, of small outside diameter and the triply-coile'd middle portion 12 of larger outside diameter. The otherwise free ends of the legs 10 are welded to the lead-in wires, the end of leg 10 being welded to lead-in wire 3 and the end of leg 11 to lead-in wire 4. vSaid wires extend upwardly from the cathode 9 in order to support the curved auxiliary anodes 13, 14, each of which is welded to a lead-in wire 3, 4, and spaced from each other.

The sealed envelope 1 is filled with an inert gas or gases, preferably a mixture of neon and 20% argon or of helium and argon as shown in an application of John F. Waymouth et al. filed concurrently herewith, at a pressure of about 1 mm. of mercury, and contains a small quantity of mercury. The latter should be sufficient to allow an operating mercury pressure of about 10 microns.

The inside of the lamp envelope 1 is preferably coated with a fluorescent material 15 in the manner customary in the art. A base 16, cemented to the end of-envelope 1, can be of a type shown in US. Patent 2,771,589, issued November 20, 1956, to R. B. Thomas. The metal shell 17 of the base 16 carries an outwardly extending hollow insulating portion 18 and the metal contacts (not shown) which are attached in the usual manner to the lead-in wires 3, 4. Other types of base can be used.

The mount structure of Figure 1 is shown also in Figure 2, in a view looking down on the auxiliary anodes 13, 14.

The filamentary structure of cathode 9 is shown in more detail in Figure 3. A fine tungsten wire is coiled loosely around a tungsten mandrel, the coil and mandrel being then coiledtogether to form a second coil, that second coil being then coiled into a third coil of larger diameter in the middle portion'--12'of,the filamentary structure; 9. The middle portion 12 carries an electron emitting filling 21 of'the usual alkaline earth oxides-withinthe turns of the first and second coilsbutnotwithin the turns of .the

third coil, the coil of largest diameter. This alkaline earth filling 16 is carried only by the middle or triply-' coiled portion of the coil. The, outwardly extending doubly-coiled legs and'll are covered with a coating of aluminum oxide 17, 18,1except for the end portions. 19, 20, which are left uncoated so'that theymay be' welded to the lead-in Wires 3, 4. The alkaline earth filling extends for a slight distance over the aluminum oxide coating to insure that there will be no gap between them.

The lead-in wires 3, 4, are covered along their straight portions by the insulating ceramic tubes 26, 27, which have the small cut--away portions 28, 29, which expose the lead-in wires at the points to which the ends 18, 19, of the cathode 12 are welded.

For clarity, only one end of the envelope 1 is shown in the figure, but the other end would be the same. 7

In operation, a voltage is applied across the lead-in wires 3, 4, to heat the cathode 12, and a larger voltage is applied between one of the lead-in wires, say wire 3, at one end of the envelope, and one'of the lead-in wires at the other end, in series with the usual ballast impedance. A glow discharge is establishedthrough the gas between the electrodes, until the cathode 1 2 becomes sufficiently heated, whereupon the discharge changes 'to an arc. When the voltage is positive at the end shown, much of the current will go to the auxiliary anodes 13, 14, but when the end shown is negative the discharge will emanate from the cathode, 12. The anodes 13, l4,are placed so that they will then be inthe EaradayDark Space of the discharge, to avoid excessive disintegration by ion bombardment. I I

The lead-in wires 3, 4, have to go through regions where greater disintegration could occur and are protected by insulating tubes. 7 v

The voltage drop at the cathode is kept to a minimum by confining the effective or active portion of the cathode to its middle portion, spacing the lead-in wires so far apart that they are outside the cathode ionization layer and collect a smaller number of ions from that layer, and connecting .the middle portion of the cathode to' the lead-in wires through coiled legs of small diameter extending outwardly from the cathode and coated with a firmly-adherent insulating material, for example, cataphoretically-deposited aluminum oxide. The extending legs also serve to reduce heat conduction from the middle portion of the cathode, and to thereby keep the effective portion at a more uniform temperature.

The small diameter of the connection between the middle portion of the cathode and the lead-in wires keeps at a minimum the number of ions which recombine there. That effect could also be achieved by extending a solid, small-diameter, lead-in wire to the middle portion of the cathode, instead of the coiled legs, but that would not be as effective in reducing losses by heat conduction.

In one embodiment of the invention, the lead-in Wires 3, 4, were of nickel, 50 mils diameter and 22 mm. apart, between centers. The shield was of S-mil nickel, about 26 mm. diameter, and was about 44 mm. from the flare, the lead-in wires 3, 4, were bent inward to a spacing of 12 mm. between centers. The distance from the center the outside circumference, to make room for the welded connection between filament legs and lead-in wires.

Figure 3 gives an outside view of the coated filament. The filament itself is a tungsten. wire about 0.0122 mil diameter coiled to an approximate ellipse of about 10.4 mils minor diameterand 16,0 mils major diameter, by being wound on two mandrels placed side by side, each being of about 5.5 mils diameter. One mandrel is of molybdenum and'is removed after the coiling, but the other mandrel of tungsten remains in place.' This coil is then wound around a 12.0 mil diameter mandrel to form a second coil, and that rnandrel removed in the usual manner. This double coil is used as the legs 10, 11, of the filamentary coil 9. For middle portion 9 of the cathode, the double-coil is itself coiled again around a 55.0 steel pin, whichis then removed, to form a fourturn triple-coil.

A tungsten insert of 0.008 mm. diameter and 8 mm. length is inserted in each end of the coil to facilitate welding to the lead-in wires 3, 4. The insert goes into the second coiling.

The aluminum oxidecoating is applied cataphoretically, as shown, for example, in copending US. patent application Serial No. 736,994, filed May 22, 1958, by

side, but also fills the coils of the first and second coil-- ing, except for the space occupied by the mandrel and the insert. The aluminum coating bridges all turns of the part it coats and fills. A spray coatingcan be used to achieve the same results. 7

After the aluminum coating is applied, it is sintered at a temperature of about 1700f C., and the cathode is welded to the lead-in wires and a coating of the usual alkaline earth oxides applied in the usual manner by dipping or the like in a coating suspension. If titanium dioxide is used, the sintering can be done at a lower temperature of about 1100 C. a

The outside envelope 1 was about 48 inches from end to end, the anodes 13, 14, at one end being about 38 /2 inches from the corresponding anodes at the other end. Theenvclope was of so-called T-12 tubing, about 1 /2 inch outside diameter and about 30 to 36 mils in thickmess. The envelope was filled with an atmosphere of neon and 20% argon at a pressure of about 1 mm. of mercury, and with at least enough mercury to give a pressure of about 10 microns when vaporized.

The lamp can be operated for lighting purposes from a so called rapid start circuit, for example, by a transformer supplying about 3.6 volts to the filaments and about 300 volts for application, through the usual series reactance, between the filament at one end of the tube and the filament at the other. The operating conditions are about 89 volts and 1.4 amperes.

Although described above with reference to a fluorescent lamp of very high output, my invention is not limited to such an embodiment, but can be used on lamps of various other types and outputs, as will be clear to one skilled in the art after reading the present specification.

A filamentary coil 9, having an electron-emitting middle portion 12 and a coating of refractory insulating material on the outwardly extending legs 10, 11, can be used as the electrode in an ordinary argon-filled fluorescent lamp of the type'in which the coil 9 is supported between lead-in wireswhich do not extend'apprec iably beyond said coil into the discharge, in which "the auxiliary anodes 1 3, 14,

are not present, and in which theinsulating tubes 26 2'1, are omitted, as they are not generally necessary when" the filling is argon.

Under such conditions, the argon pressure can be made lower than is customary in such lamps. It may, for example, have a pressure of 1 mm. of mercury, thereby increasing the light output and efliciency of the lamp, without lowering its life. Without the insulating coating on legs 10, 11, the lamp life would be reduced at low pressure.

What I claim is:

1. In an electric discharge lamp, a pair of lead-in wires and an electrode therebetween said electrode comprising a coiled refractory wire having a middle portion and end portions, a coating of electron-emissive material on the middle portion and a coating of firmly-adherent refractory insulating material covering substantially the entire distance along said end portions between said lead-in wires and said middle portion.

2. An electric discharge lamp comprising a sealed, light-transmitting envelope, a filling of mercury vapor and inert gas therein at low pressure, a pair of lead-in wires at each end of said envelope, and a coiled refractory wire extending from one wire of said pair to the other wire, said coiled wire having a middle portion and end portions, a coating of electron-emissive material on the middle portion and a coating of firmly-adherent refractory insulating material covering substantially the entire distance along said end-portion between said lead-in Wires and said middle portion.

3. The combination of claim 2, in which the middle portion of the coiled wire is triply-coiled and the end portions only doubly-coiled.

4. The combination of claim 3, in which the refractory insulating material fills the space inside the doubly-coiled portion as well as extending over the outside of the doubly-coiled portion.

References Cited in the file of this patent UNITED STATES PATENTS Germer June 22, 1954 Kolkman Jan. 31, 1956 

